Resist composition and patterning process

ABSTRACT

A resist composition comprising a base polymer and a sulfonium salt of iodized benzoic acid offers a high sensitivity and minimal LWR independent of whether it is of positive or negative tone.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 15/598,428, filed on May 18, 2017, and wherein U.S. application Ser. No. 15/598,428 is a non-provisional application claiming priority under 35 U.S.C. § 119(a) on Patent Application No. 2016-113475 filed in Japan on Jun. 7, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a resist composition comprising a sulfonium salt capable of generating iodized benzoic acid and a pattern forming process.

BACKGROUND ART

To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, EUV lithography of wavelength 13.5 nm, and double patterning version of the ArF lithography, on which active research efforts have been made.

Chemically amplified resist compositions comprising an acid generator capable of generating an acid upon exposure to light or EB include chemically amplified positive resist compositions wherein deprotection reaction takes place under the action of acid and chemically amplified negative resist compositions wherein crosslinking reaction takes place under the action of acid. Quenchers are often added to these resist compositions for the purpose of controlling the diffusion of the acid to unexposed areas to improve the contrast. The addition of quenchers is fully effective to this purpose. A number of amine quenchers were proposed as disclosed in Patent Documents 1 to 3.

As the pattern feature size is reduced, approaching to the diffraction limit of light, light contrast lowers. In the case of positive resist film, a lowering of light contrast leads to reductions of resolution and focus margin of hole and trench patterns.

For mitigating the influence of reduced resolution of resist pattern due to a lowering of light contrast, an attempt is made to enhance the dissolution contrast of resist film. One such attempt is a chemically amplified resist material utilizing an acid amplifying mechanism that a compound is decomposed with an acid to generate another acid. In general, the concentration of acid creeps up linearly with an increase of exposure dose. In the case of the acid amplifying mechanism, the concentration of acid jumps up non-linearly as the exposure dose increases. The acid amplifying system is beneficial for further enhancing the advantages of chemically amplified resist film including high contrast and high sensitivity, but worsens the drawbacks of chemically amplified resist film that environmental resistance is degraded by amine contamination and maximum resolution is reduced by an increase of acid diffusion distance. The acid amplifying system is very difficult to control when implemented in practice.

Another approach for enhanced contrast is by reducing the concentration of amine with an increasing exposure dose. This may be achieved by applying a compound which loses the quencher function upon light exposure.

With respect to the acid labile group used in methacrylate polymers for the ArF lithography, deprotection reaction takes place when a photoacid generator capable of generating a sulfonic acid having fluorine substituted at α-position (referred to “α-flourinated sulfonic acid”) is used, but not when an acid generator capable of generating a sulfonic acid not having fluorine substituted at α-position (referred to “α-non-fluorinated sulfonic acid”) or carboxylic acid is used. If a sulfonium or iodonium salt capable of generating an α-flourinated sulfonic acid is combined with a sulfonium or iodonium salt capable of generating an α-non-fluorinated sulfonic acid, the sulfonium or iodonium salt capable of generating an α-non-fluorinated sulfonic acid undergoes ion exchange with the α-flourinated sulfonic acid. Through the ion exchange, the α-flourinated sulfonic acid thus generated by light exposure is converted back to the sulfonium or iodonium salt while the sulfonium or iodonium salt of an α-non-fluorinated sulfonic acid or carboxylic acid functions as a quencher.

Further, the sulfonium or iodonium salt capable of generating an α-non-fluorinated sulfonic acid also functions as a photodegradable quencher since it loses the quencher function by photodegradation. Non-Patent Document 1 points out that the addition of a photodegradable quencher expands the margin of a trench pattern although the structural formula is not illustrated. However, it has only a little influence on performance improvement. There is a desire to have a quencher for further improving contrast.

Patent Document 4 discloses a quencher of onium salt type which reduces its basicity through a mechanism that it generates an amino-containing carboxylic acid upon light exposure, which in turn forms a lactam in the presence of acid. Due to the mechanism that basicity is reduced under the action of acid, acid diffusion is controlled by high basicity in the unexposed region where the amount of acid generated is minimal, whereas acid diffusion is promoted due to reduced basicity of the quencher in the overexposed region where the amount of acid generated is large. This expands the difference in acid amount between the exposed and unexposed regions, from which an improvement in contrast is expected. Despite the advantage of improved contrast, the acid diffusion controlling effect is rather reduced.

As the pattern feature size is reduced, edge roughness (LWR) is regarded significant. It is pointed out that LWR is affected by the segregation or agglomeration of a base polymer and acid generator and the diffusion of generated acid. There is a tendency that as the resist film becomes thinner, LWR becomes greater. A film thickness reduction to comply with the progress of size reduction causes a degradation of LWR, which becomes a serious problem.

The EUV lithography resist must meet high sensitivity, high resolution and low LWR at the same time. As the acid diffusion distance is reduced, LWR is reduced, but sensitivity becomes lower. For example, as the PEB temperature is lowered, the outcome is a reduced LWR, but a lower sensitivity. As the amount of quencher added is increased, the outcome is a reduced LWR, but a lower sensitivity. It is necessary to overcome the tradeoff relation between sensitivity and LWR.

CITATION LIST

Patent Document 1: JP-A 2001-194776

Patent Document 2: JP-A 2002-226470

Patent Document 3: JP-A 2002-363148

Patent Document 4: JP-A 2015-090382

Non-Patent Document 1: SPIE Vol. 7639 p76390W (2010)

DISCLOSURE OF INVENTION

For the acid-catalyzed chemically amplified resist, it is desired to develop a quencher capable of providing a high sensitivity and reducing LWR.

An object of the invention is to provide a resist composition which exhibits a high sensitivity and a reduced LWR, independent of whether it is of positive tone or negative tone; and a pattern forming process using the same.

The inventors have found that using a sulfonium salt of iodized benzoic acid as the quencher, a photoresist material having a reduced LWR, high contrast, improved resolution, and wide process margin is obtainable.

In one aspect, the invention provides a resist composition comprising a base polymer and a sulfonium salt having the formula (A).

Herein R¹ is hydroxyl, a C₁-C₆ straight, branched or cyclic alkyl or alkoxy group, C₂-C₆ straight, branched or cyclic acyloxy group, or C₁-C₄ straight, branched or cyclic alkylsulfonyloxy group, in which at least one hydrogen may be substituted by a halogen atom, or fluorine, chlorine, bromine, amino group, nitro group, cyano group, —NR⁵—C(═O)—R⁶, or —NR⁵—C(═O)—O—R⁶, wherein R⁵ is hydrogen or a C₁-C₆ straight, branched or cyclic alkyl group, R⁶ is a C₁-C₆ straight, branched or cyclic alkyl group or C₂-C₈ straight, branched or cyclic alkenyl group. X is a single bond or a (p+1)-valent C₁-C₂₀ linking group which may contain an ether, carbonyl, ester, amide, sultone, lactam, carbonate, halogen, hydroxy or carboxy moiety. R², R³ and R⁴ are each independently fluorine, chlorine, bromine, iodine, or a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain an oxo moiety, a C₂-C₁₂ straight, branched or cyclic alkenyl group which may contain an oxo moiety, C₆-C₂₀ aryl group, C₇-C₁₂ aralkyl group or C₇-C₁₂ aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R² and R³ may bond together to form a ring with the sulfur atom to which they are attached, m is an integer of 1 to 5, n is an integer of 0 to 3, and p is an integer of 1 to 3. Preferably m is equal to 3.

The resist composition may further comprise an acid generator capable of generating a sulfonic acid, imide acid or methide acid, and/or an organic solvent.

In a preferred embodiment, the base polymer comprises recurring units having the formula (al) or recurring units having the formula (a2).

Herein R^(A) is each independently hydrogen or methyl, Y¹ is a single bond, ester group, C₁-C₁₂ linking group containing lactone ring, phenylene group or naphthylene group, Y² is a single bond or ester group, R¹¹ and R¹² each are an acid labile group.

The resist composition may further comprise a dissolution inhibitor.

In one embodiment, the resist composition is a chemically amplified positive resist composition.

In another preferred embodiment, the resist composition is a chemically amplified negative resist composition. Typically the base polymer is free of an acid labile group. The resist composition may further comprise a crosslinker.

The resist composition may further comprise a surfactant.

In a preferred embodiment, the base polymer further comprises recurring units of at least one type selected from the formulae (f1) to (f3).

Herein R^(A) is each independently hydrogen or methyl; R²¹ is a single bond, phenylene group, —O—R³¹—, or —C(═O)-Q¹-R³¹—, Q¹ is —O— or —NH—, R³¹ is a C₁-C₆ straight, branched or cyclic alkylene group, C₂-C₆ straight, branched or cyclic alkenylene group, or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety; R²² to R²⁹ are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C₆-C₁₂ aryl group, C₇-C₂₀ aralkyl group or to mercaptophenyl group; Z¹ is a single bond, —Z¹¹—C(═O)—O—, Z¹¹—O— or —Z¹¹—O—C(═(O)—, Z¹¹ is a C₁-C₁₂ straight, branched or cyclic alkylene group which may contain a carbonyl, ester or ether moiety; A is hydrogen or trifluoromethyl; Z² is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R³²—, or —C(═O)—Z¹²—R³²—, Z¹² is —O— or —NH—, R³² is a C₁-C₆ straight, branched or cyclic alkylene group, phenylene group, fluorinated phenylene group, trifluoromethyl-substituted phenylene group, or C₂-C₆ straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxy moiety; M⁻ is a non-nucleophilic counter ion.

In another aspect, the invention provides a process for forming a pattern comprising the steps of applying the resist composition defined above onto a substrate, baking to form a resist film, exposing the resist film to high-energy radiation, and developing the exposed film in a developer.

Preferably, the high-energy radiation is ArF excimer laser radiation of wavelength 193 nm, KrF excimer laser radiation of wavelength 248 nm, EB, or EUV of wavelength 3 to 15 nm.

In a further aspect, the invention provides a sulfonium salt having the formula (A-1).

Herein R^(1′) is iodine or hydroxyl; R^(2′), R^(3′) and R^(4′) are each independently a C_(i)-C₁₂ straight, branched or cyclic alkyl group which may contain an oxo moiety, a C₂-C₁₂ straight, branched or cyclic alkenyl group which may contain an oxo moiety, C₆-C₂₀ aryl group, C₇-C₁₂ aralkyl group or C₇-C₁₂ aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R^(2′) and R^(3′) may bond together to form a ring with the sulfur atom to which they are attached.

ADVANTAGEOUS EFFECTS OF INVENTION

A resist film containing a sulfonium salt of iodized benzoic acid has the advantage that the sulfonium salt of iodized benzoic acid is highly effective for suppressing acid diffusion because of the large atomic weight of iodine. This leads to a reduced LWR. Since iodine is highly absorptive to EUV of wavelength 13.5 nm, it generates many secondary electrons during exposure, contributing to a higher sensitivity. Thus a resist material having a high sensitivity and low LWR may be designed. Using the resist material, a hole pattern of improved CDU can be formed.

DESCRIPTION OF EMBODIMENTS

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. The notation (C_(n)-C_(m)) means a group containing from n to m carbon atoms per group. As used herein, the term “iodized” compound means an iodine-containing compound. In chemical formulae, Me stands for methyl, Ac for acetyl, and Ph for phenyl.

The abbreviations and acronyms have the following meaning.

EB: electron beam

EUV: extreme ultraviolet

Mw: weight average molecular weight

Mn: number average molecular weight

Mw/Mn: molecular weight distribution or dispersity

GPC: gel permeation chromatography

PEB: post-exposure bake

PAG: photoacid generator

LWR: line width roughness

CDU: critical dimension uniformity

Resist composition

The resist composition of the invention is defined as comprising a base polymer and a sulfonium salt of iodized benzoic acid. Although the sulfonium salt is an acid generator capable of generating an iodized benzoic acid upon light exposure, it functions as a quencher because it has a strongly basic sulfonium cation. Since the iodized benzoic acid does not possess a sufficient acidity to induce deprotection reaction of an acid labile group, it is recommended to separately add an acid generator capable of generating a strong acid such as α-flourinated sulfonic acid, imide acid or methide acid, as will be described later, in order to induce deprotection reaction of an acid labile group. The acid generator capable of generating α-flourinated sulfonic acid, imide acid or methide acid may be either of separate type which is added to the base polymer or of bound type which is bound in the base polymer.

When a resist composition containing the sulfonium salt of iodized benzoic acid in admixture with an acid generator capable of generating a perfluoroalkylsulfonic acid or superstrong acid is exposed to radiation, iodized benzoic acid and perfluoroalkylsulfonic acid generate. Since the acid generator is not entirely decomposed, the undecomposed acid generator is present nearby. When the sulfonium salt capable of generating iodized benzoic acid co-exists with the perfluoroalkylsulfonic acid, first the perfluoroalkylsulfonic acid undergoes ion exchange with the iodized benzoic acid sulfonium salt, whereby a sulfonium salt of perfluoroalkylsulfonic acid is created and an iodized benzoic acid is released. This is because the salt of perfluoroalkylsulfonic acid having a high acid strength is more stable. In contrast, when a sulfonium salt of perfluoroalkylsulfonic acid co-exists with an iodized benzoic acid, no ion exchange takes place. The ion exchange reaction according to the acid strength series occurs not only with sulfonium salts, but also similarly with iodonium salts. Likewise, ion exchange takes place not only with the perfluoroalkylsulfonic acid, but also similarly with arylsulfonic acid, alkylsulfonic acid, imide acid and methide acid having a higher acid strength than the iodized benzoic acid.

An iodized benzoic acid has a larger molecular weight than unsubstituted benzoic acid and thus a higher ability to suppress acid diffusion. Since iodine is highly absorptive to EUV of wavelength 13.5 nm, it generates secondary electrons upon EUV exposure. The energy of secondary electrons is transferred to the acid generator to promote its decomposition, contributing to a higher sensitivity. The effect becomes significant when the number of iodine substitution is 3 or more.

While the resist composition of the invention should essentially contain the sulfonium salt of iodized benzoic acid, another sulfonium or iodonium salt may be separately added as the quencher. Examples of the sulfonium or iodonium salt to be added as the quencher include sulfonium or iodonium salts of carboxylic acid, sulfonic acid, imide acid and saccharin. The carboxylic acid used herein may or may not be fluorinated at α-position.

For the LWR improving purpose, it is effective to prevent a polymer and/or acid generator from agglomeration as indicated above. Effective means for preventing agglomeration of a polymer is by reducing the difference between hydrophobic and hydrophilic properties or by lowering the glass transition temperature (Tg) thereof. Specifically, it is effective to reduce the polarity difference between a hydrophobic acid labile group and a hydrophilic adhesive group or to lower the Tg by using a compact adhesive group like monocyclic lactone. One effective means for preventing agglomeration of an acid generator is by introducing a substituent into the triphenylsulfonium cation. In particular, with respect to a methacrylate polymer containing an alicyclic protective group and a lactone adhesive group for ArF lithography, a triphenylsulfonium composed solely of aromatic groups has a heterogeneous structure and low compatibility. As the substituent to be introduced into triphenylsulfonium, an alicyclic group or lactone similar to those used in the base polymer is regarded adequate. When lactone is introduced in a sulfonium salt which is hydrophilic, the resulting sulfonium salt becomes too hydrophilic and thus less compatible with a polymer, with a likelihood that the sulfonium salt will agglomerate. When a hydrophobic alkyl group is introduced, the sulfonium salt may be uniformly dispersed within the resist film. WO 2011/048919 discloses the technique for improving LWR by introducing an alkyl group into a sulfonium salt capable of generating an α-flourinated sulfone imide acid.

Another factor to be taken for the LWR improving purpose is the dispersibility of a quencher. Even when the dispersibility of an acid generator in a resist film is improved, the uneven distribution of a quencher can cause a degradation of LWR. For those quenchers of sulfonium salt type, the introduction of an alkyl or similar substituent into the triphenylsulfonium cation moiety is effective for LWR improvement. Further, the introduction of halogen atoms into the quencher of sulfonium salt type efficiently enhances hydrophobicity and improves dispersibility. The introduction of bulky halogen atoms such as iodine is effective not only in the cation moiety, but also in the anion moiety of the sulfonium salt. The sulfonium salt of iodized benzoic acid according to the invention has iodine introduced in the anion moiety, whereby the dispersibility of the quencher within the resist film is enhanced and LWR is reduced.

The sulfonium salt of iodized benzoic acid exerts a LWR reducing effect, which may stand good either in positive and negative tone pattern formation by alkaline development or in negative tone pattern formation by organic solvent development.

Sulfonium Salt

The sulfonium salt in the resist composition has the formula (A).

In formula (A), R¹ is hydroxyl, or a C₁-C₆ straight, branched or cyclic alkyl or alkoxy group, C₂-C₆ straight, branched or cyclic acyloxy group, or C₁-C₄ straight, branched or cyclic alkylsulfonyloxy group, in which at least one hydrogen (one or more or even all hydrogen atoms) may be substituted by a halogen atom, or fluorine, chlorine, bromine, amino group, nitro group, cyano group, —NR⁵—C(═O)—R⁶, or —NR⁵—C(═O)—O—R⁶, wherein R⁵ is hydrogen or a C₁-C₆ straight, branched or cyclic alkyl group, R⁶ is a C₁-C₆ straight, branched or cyclic alkyl group or C₂-C₈ straight, branched or cyclic alkenyl group. X is a single bond or a (p+1)-valent C₁-C₂₀ linking group which may contain an ether, carbonyl, ester, amide, sultone, lactam, carbonate, halogen, hydroxy or carboxy moiety. R², R³ and R⁴ are each independently fluorine, chlorine, bromine, iodine, or a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain an oxo moiety, a C₂-C₁₂ straight, branched or cyclic alkenyl group which may contain an oxo moiety, C₆-C₂₀ aryl group, C₇-C₁₂ aralkyl group or C₇-C₁₂ aryloxyalkyl group, in which at least one hydrogen (one or more or even all hydrogen atoms) may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R² and R³ may bond together to form a ring with the sulfur atom to which they are attached, m is an integer of 1 to 5, n is an integer of 0 to 3, and p is an integer of 1 to 3.

Of the sulfonium salts, those of the formula (A-1) having a high iodine density are preferred whereby high sensitivity and low LWR or better CDU are achieved.

Herein R^(1′) is iodine or hydroxyl. R^(2′), R^(3′) and R^(4′) are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain an oxo moiety, a C₂-C₁₂ straight, branched or cyclic alkenyl group which may contain an oxo moiety, C₆-C₂₀ aryl group, C₇-C₁₂ aralkyl group or C₇-C₁₂ aryloxyalkyl group, in which at least one hydrogen (one or more or even all hydrogen atoms) may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms. R^(2′) and R^(3′) may bond together to form a ring with the sulfur atom to which they are attached.

Examples of the cation moiety in the sulfonium salt having formula (A) are given below, but not limited thereto.

Examples of the anion moiety in the sulfonium salt having formula (A) are given below, but not limited thereto.

The sulfonium salt of iodized benzoic acid having formula (A) may be synthesized, for example, by ion exchange with a sulfonium salt of weaker acid than the iodized benzoic acid. Typical of the weaker acid than the iodized benzoic acid is carbonic acid. Alternatively, the sulfonium salt may be synthesized by ion exchange of a sodium salt of an iodized benzoic acid with a sulfonium chloride.

In the resist composition, the sulfonium salt having formula (A) is preferably used in an amount of 0.001 to 50 parts, more preferably 0.01 to 20 parts by weight per 100 parts by weight of the base polymer, as viewed from sensitivity and acid diffusion suppressing effect.

Base Polymer

Where the resist composition is of positive tone, the base polymer comprises recurring units containing an acid labile group, preferably recurring units having the formula (a1) or recurring units having the formula (a2). These units are simply referred to as recurring units (a1) and (a2).

Herein R^(A) is each independently hydrogen or methyl. Y¹ is a single bond, ester group, C₁-C₁₂ linking group containing lactone ring, phenylene group or naphthylene group. Y² is a single bond or ester group. R¹¹ and R¹² each are an acid labile group. When the base polymer contains both recurring units (al) and (a2), R¹¹ and R¹² may be the same or different.

Examples of the recurring units (al) are shown below, but not limited thereto. R^(A) and R¹¹ are as defined above.

The acid labile groups represented by R¹¹ and R¹² in the recurring units (a1) and (a2) may be selected from a variety of such groups, for example, those groups described in JP-A 2013-080033 (U.S. Pat. No. 8,574,817) and JP-A 2013-083821 (U.S. Pat. No. 8,846,303).

Typical of the acid labile group are groups of the following formulae (AL-1) to (AL-3).

In formulae (AL-1) and (AL-2), R¹³ and R¹⁶ are each independently a monovalent hydrocarbon group of 1 to 40 carbon atoms, preferably 1 to 20 carbon atoms, typically straight, branched or cyclic alkyl, which may contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine. R¹⁴ and R¹⁵ are each independently hydrogen or a monovalent hydrocarbon group of 1 to 20 carbon atoms, typically straight, branched or cyclic alkyl, which may contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine. A1 is an integer of 0 to 10, especially 1 to 5. A pair of R¹⁴ and R¹⁵, R¹⁴ and R¹⁶, or R¹⁵ and R¹⁶ may bond together to form a ring, typically alicyclic, with the carbon atom or carbon and oxygen atoms to which they are attached, the ring containing 3 to 20 carbon atoms, preferably 4 to 16 carbon atoms.

In formula (AL-3), R¹⁷, R¹⁸ and R¹⁹ are each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms, typically straight, branched or cyclic alkyl, which may contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine. A pair of R¹⁷ and R¹⁸, R¹⁷ and R¹⁹, or R¹⁸ and R¹⁹ may bond together to form a ring, typically alicyclic, with the carbon atom to which they are attached, the ring containing 3 to 20 carbon atoms, preferably 4 to 16 carbon atoms.

The base polymer may further comprise recurring units (b) having a phenolic hydroxyl group as an adhesive group. Examples of suitable monomers from which recurring units (b) are derived are given below, but not limited thereto. Herein R^(A) is as defined above.

Further, recurring units (c) having another adhesive group selected from hydroxyl (other than the foregoing phenolic hydroxyl), lactone ring, ether, ester, carbonyl and cyano groups may also be incorporated in the base polymer. Examples of suitable monomers from which recurring units (c) are derived are given below, but not limited thereto. Herein R^(A) is as defined above.

In the case of a monomer having a hydroxyl group, the hydroxyl group may be replaced by an acetal group susceptible to deprotection with acid, typically ethoxyethoxy, prior to polymerization, and the polymerization be followed by deprotection with weak acid and water. Alternatively, the hydroxyl group may be replaced by an acetyl, formyl, pivaloyl or similar group prior to polymerization, and the polymerization be followed by alkaline hydrolysis.

In another preferred embodiment, the base polymer may further comprise recurring units (d) selected from units of indene, benzofuran, benzothiophene, acenaphthylene, chromone, coumarin, and norbornadiene, or derivatives thereof. Suitable monomers are exemplified below.

Besides the recurring units described above, further recurring units (e) may be incorporated in the base polymer, examples of which include styrene, vinylnaphthalene, vinylanthracene, vinylpyrene, methyleneindene, vinylpyridine, and vinylcarbazole.

In a further embodiment, recurring units (f) derived from an onium salt having polymerizable olefin may be incorporated in the base polymer. JP-A 2005-084365 discloses sulfonium and iodonium salts having polymerizable olefin capable of generating a sulfonic acid. JP-A 2006-178317 discloses a sulfonium salt having sulfonic acid directly attached to the main chain.

In a preferred embodiment, the base polymer may further comprise recurring units of at least one type selected from formulae (f1), (f2) and (f3). These units are simply referred to as recurring units (f1), (f2) and (f3), which may be used alone or in combination of two or more types.

Herein R^(A) is each independently hydrogen or methyl. R²¹ is a single bond, phenylene group, —O—R³¹—, or —C(═O)-Q¹-R³¹—, wherein Q¹ is —O— or —NH—, R³¹ is a C₁-C₆ straight, branched or cyclic alkylene group, C₂-C₆ straight, branched or cyclic alkenylene group, or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety. R²² to R²⁹ are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C₆-C₁₂ aryl group, C₇-C₂₀ aralkyl group or mercaptophenyl group. Z¹ is a single bond, —Z¹¹—C(═O)—O—, —Z¹¹—O— or —Z¹¹—O—C(═O)—, wherein Z¹¹ is a C₁-C₁₂ straight, branched or cyclic alkylene group which may contain a carbonyl, ester or ether moiety. A is hydrogen or trifluoromethyl. Z² is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R³²—, or —C(═O)—Z¹²—R³²—, wherein Z¹² is —O— or —NH—, R³² is a C₁-C₆ straight, branched or cyclic alkylene group, phenylene group, fluorinated phenylene group, trifluoromethyl-substituted phenylene group, or C₂-C₆ straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxy moiety. M⁻ is a non-nucleophilic counter ion.

Examples of the monomer from which recurring unit (f1) is derived are shown below, but not limited thereto. R^(A) and M⁻ are as defined above.

Examples of the non-nucleophilic counter ion M⁻ include halide ions such as chloride and bromide ions; fluoroalkylsulfonate ions such as triflate, 1,1,1-trifluoroethanesulfonate, and nonafluorobutanesulfonate; arylsulfonate ions such as tosylate, benzenesulfonate, 4-fluorobenzenesulfonate, and 1,2,3,4,5-pentafluorobenzenesulfonate; alkylsulfonate ions such as mesylate and butanesulfonate; imidates such as bis(trifluoromethylsulfonyl)imide, bis(perfluoroethylsulfonyl)imide and bis(perfluorobutylsulfonyl)imide; methidates such as tris(trifluoromethylsulfonyl)methide and tris(perfluoroethylsulfonyl)methide.

Also included are sulfonates having fluorine substituted at α-position as represented by the formula (K-1) and sulfonates having fluorine substituted at α- and β-positions as represented by the formula (K-2).

In formula (K-1), R⁵¹ is hydrogen, or a C₁-C₂₀ straight, branched or cyclic alkyl group, C₂-C₂₀ straight, branched or cyclic alkenyl group, or C₆-C₂₀ aryl group, which may contain an ether, ester, carbonyl moiety, lactone ring, or fluorine atom. In formula (K-2), R⁵² is hydrogen, or a C₁-C₃₀ straight, branched or cyclic alkyl group, C₂-C₃₀ straight, branched or cyclic acyl group, C₂-C₂₀ straight, branched or cyclic alkenyl group, C₆-C₂₀ aryl group or C₆-C₂₀ aryloxy group, which may contain an ether, ester, carbonyl moiety or lactone ring.

Examples of the monomer from which recurring unit (f2) is derived are shown below, but not limited thereto. R^(A) is as defined above.

Examples of the monomer from which recurring unit (f3) is derived are shown below, but not limited thereto. R^(A) is as defined above.

The attachment of an acid generator to the polymer main chain is effective in restraining acid diffusion, thereby preventing a reduction of resolution due to blur by acid diffusion. Also edge roughness is improved since the acid generator is uniformly distributed. Where a base polymer containing recurring units (f) is used, the addition of a separate PAG may be omitted.

The base polymer for formulating the positive resist composition comprises recurring units (al) or (a2) having an acid labile group as essential component and additional recurring units (b), (c), (d), (e), and (f) as optional components. A fraction of units (a1), (a2), (b), (c), (d), (e), and (f) is: preferably 0≤a1<1.0, 0≤a2<1.0, 0<a1+a2<1.0, 0≤b≤0.9, 0≤c≤0.9, 0≤d≤0.8, 0≤e≤0.8, and 0≤f≤0.5; more preferably 0≤a1≤0.9, 0≤a2≤0.9, 0.1≤a1+a2≤0.9, 0≤b≤0.8, 0≤c≤0.8, 0≤d≤0.7, 0≤e≤0.7, and 0≤f≤0.4; and even more preferably 0≤a≤0.8, 0≤a2≤0.8, 0.1≤a1+a2≤0.8, 0≤b≤0.75, 0≤c≤0.75, 0≤d≤0.6, 0≤e≤0.6, and 0≤f≤0.3. Notably, f=f1+f2+f3, meaning that unit (f) is at least one of units (f1) to (f3), and a1+a2+b+c+d+e+f=1.0.

For the base polymer for formulating the negative resist composition, an acid labile group is not necessarily essential. The base polymer comprises recurring units (b), and optionally recurring units (c), (d), (e), and/or (f). A fraction of these units is: preferably 0<b≤1.0, 0≤c≤0.9, 0≤d≤0.8, 0≤e≤0.8, and 0≤f≤0.5; more preferably 0.2≤b≤1.0, 0≤c≤0.8, 0≤d≤0.7, 0≤e≤0.7, and 0≤f≤0.4; and even more preferably 0.3≤b≤1.0, 0≤c≤0.75, 0≤d≤0.6, 0≤e≤0.6, and 0≤f≤0.3. Notably, f=f1+f2+f3, meaning that unit (f) is at least one of units (f1) to (f3), and b+c+d+e+f =1.0.

The base polymer may be synthesized by any desired methods, for example, by dissolving one or more monomers selected from the monomers corresponding to the foregoing recurring units in an organic solvent, adding a radical polymerization initiator thereto, and effecting heat polymerization. Examples of the organic solvent which can be used for polymerization include toluene, benzene, tetrahydrofuran, diethyl ether, and dioxane. Examples of the polymerization initiator used herein include 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2,4-dimethylvaleronitrile), dimethyl 2,2-azobis(2-methylpropionate), benzoyl peroxide, and lauroyl peroxide. Preferably the system is heated at 50 to 80° C. for polymerization to take place. The reaction time is 2 to 100 hours, preferably 5 to 20 hours.

When hydroxystyrene or hydroxyvinylnaphthalene is copolymerized, an alternative method is possible. Specifically, acetoxystyrene or acetoxyvinylnaphthalene is used instead of hydroxystyrene or hydroxyvinylnaphthalene, and after polymerization, the acetoxy group is deprotected by alkaline hydrolysis, for thereby converting the polymer product to hydroxystyrene or hydroxyvinylnaphthalene. For alkaline hydrolysis, a base such as aqueous ammonia or triethylamine may be used. Preferably the reaction temperature is −20° C. to 100° C., more preferably 0° C. to 60° C., and the reaction time is 0.2 to 100 hours, more preferably 0.5 to 20 hours.

The base polymer should preferably have a weight average molecular weight (Mw) in the range of 1,000 to 500,000, and more preferably 2,000 to 30,000, as measured by GPC versus polystyrene standards using tetrahydrofuran (THF) solvent. With too low a Mw, the resist composition may become less heat resistant. A polymer with too high a Mw may lose alkaline solubility and give rise to a footing phenomenon after pattern formation.

If a base polymer has a wide molecular weight distribution or dispersity (Mw/Mn), which indicates the presence of lower and higher molecular weight polymer fractions, there is a possibility that foreign matter is left on the pattern or the pattern profile is degraded. The influences of molecular weight and dispersity become stronger as the pattern rule becomes finer. Therefore, the base polymer should preferably have a narrow dispersity (Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide a resist composition suitable for micropatterning to a small feature size.

It is understood that a blend of two or more polymers which differ in compositional ratio, Mw or Mw/Mn is acceptable.

Acid Generator

To the resist composition comprising the base polymer and the sulfonium salt having formula (A), an acid generator may be added so that the composition may function as a chemically amplified positive or negative resist composition. The acid generator is typically a compound (PAG) capable of generating an acid upon exposure to actinic ray or radiation. Although the PAG used herein may be any compound capable of generating an acid upon exposure to high-energy radiation, those compounds capable of generating sulfonic acid, imide acid (imidic acid) or methide acid are preferred. Suitable PAGs include sulfonium salts, iodonium salts, sulfonyldiazomethane, N-sulfonyloxyimide, and oxime-O-sulfonate acid generators. Exemplary PAGs are described in JP-A 2008-111103, paragraphs [0122]-[0142] (U.S. Pat. No. 7,537,880).

As the PAG used herein, those having the formula (1) are preferred.

In formula (1), R¹⁰¹, R¹⁰² and R¹⁰³ are each independently a C₁-C₂₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. Any two of R¹⁰¹, R¹⁰² and R¹⁰³ may bond together to form a ring with the sulfur atom to which they are attached.

In formula (1), X⁻ is an anion of the following formula (1A), (1B), (1C) or (1D).

In formula (1A), R^(fa) is fluorine or a C₁-C₄₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom.

Of the anions of formula (1A), an anion having the formula (1A′) is preferred.

In formula (1A′), R¹⁰⁴ is hydrogen or trifluoromethyl, preferably trifluoromethyl. R¹⁰⁵ is a C₁-C₃₈ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. As the heteroatom, oxygen, nitrogen, sulfur and halogen atoms are preferred, with oxygen being most preferred. Of the monovalent hydrocarbon groups represented by R¹⁰⁵, those groups of 6 to 30 carbon atoms are preferred from the aspect of achieving a high resolution in forming patterns of fine feature size. Suitable monovalent hydrocarbon groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, s-butyl, t-butyl, pentyl, neopentyl, cyclopentyl, hexyl, cyclohexyl, 3-cyclohexenyl, heptyl, 2-ethylhexyl, nonyl, undecyl, tridecyl, pentadecyl, heptadecyl, 1-adamantyl, 2-adamantyl, 1-adamantylmethyl, norbornyl, norbornylmethyl, tricyclodecanyl, tetracyclododecanyl, tetracyclododecanylmethyl, dicyclohexylmethyl, eicosanyl, allyl, benzyl, diphenylmethyl, tetrahydrofuryl, methoxymethyl, ethoxymethyl, methylthiomethyl, acetamidomethyl, trifluoromethyl, (2-methoxyethoxy)methyl, acetoxymethyl, 2-carboxy-1-cyclohexyl, 2-oxopropyl, 4-oxo-1-adamantyl, and 3-oxocyclohexyl. In these groups, one or more hydrogen atoms may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, or a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene between carbon atoms, so that the group may contain a hydroxyl, cyano, carbonyl, ether, ester, sulfonic acid ester, carbonate, lactone ring, sultone ring, carboxylic anhydride or haloalkyl moiety.

With respect to the synthesis of the sulfonium salt having an anion of formula (1A′), reference may be made to JP-A 2007-145797, JP-A 2008-106045, JP-A 2009-007327, and JP-A 2009-258695. Also useful are the sulfonium salts described in JP-A 2010-215608, JP-A 2012-041320, JP-A 2012-106986, and JP-A 2012-153644.

Examples of the sulfonium salt having an anion of formula (1A) are shown below, but not limited thereto.

In formula (1B), R^(fb1) and R^(fb2) are each independently fluorine or a C₁-C₄₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. Illustrative examples of the monovalent hydrocarbon group are as exemplified for R¹⁰⁵. Preferably R^(fb1) and R^(fb2) are fluorine or C₁-C₄ straight fluorinated alkyl groups. Also, R^(fb1) and R^(fb2) may bond together to form a ring with the linkage: —CF₂—SO₂—N⁻—SO₂—CF₂— to which they are attached. It is preferred to form a ring structure via a fluorinated ethylene or fluorinated propylene group.

In formula (1C), R^(fc1) and R^(fc3) are each independently fluorine or a C₁-C₄₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. Illustrative examples of the monovalent hydrocarbon group are as exemplified for R¹⁰⁵. Preferably R^(fc1), R^(fc2) and R^(fc3) are fluorine or C₁-C₄ straight fluorinated alkyl groups. Also, R^(fc1) and R^(fc2) may bond together to form a ring with the linkage: —CF₂—SO₂—C⁻—SO₂—CF₂— to which they are attached. It is preferred to form a ring structure via a fluorinated ethylene or fluorinated propylene group.

In formula (1D), R^(fd) is a C₁-C₄₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. Illustrative examples of the monovalent hydrocarbon group are as exemplified for R¹⁰⁵.

With respect to the synthesis of the sulfonium salt having an anion of formula (1D), reference may be made to JP-A 2010-215608 and JP-A 2014-133723.

Examples of the sulfonium salt having an anion of formula (1D) are shown below, but not limited thereto.

Notably, the compound having the anion of formula (1D) does not have fluorine at the α-position relative to the sulfo group, but two trifluoromethyl groups at the β-position. For this reason, it has a sufficient acidity to sever the acid labile groups in the resist polymer. Thus the compound is an effective PAG.

Another preferred PAG is a compound having the formula (2).

In formula (2), R²⁰¹ and R²⁰² are each independently a C₁-C₃₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. R²⁰³ is a C₁-C₃₀ straight, branched or cyclic divalent hydrocarbon group which may contain a heteroatom. Any two of R²⁰¹, R²⁰² and R²⁰³ may bond together to form a ring with the sulfur atom to which they are attached. L^(A) is a single bond, ether bond or a C₁-C₂₀ straight, branched or cyclic divalent hydrocarbon group which may contain a heteroatom. X^(A), X^(B), X^(C) and X^(D) are each independently hydrogen, fluorine or trifluoromethyl, with the proviso that at least one of X^(A), X^(B), X^(C) and X^(D) is fluorine or trifluoromethyl, and k is an integer of 0 to 3.

Examples of the monovalent hydrocarbon group include methyl, ethyl, propyl, isopropyl, n-butyl, s-butyl, t-butyl, n-pentyl, t-pentyl, n-hexyl, n-octyl, n-nonyl, n-decyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl, cyclohexylbutyl, norbornyl, oxanorbornyl, tricyclo[5.2.1.0^(2,6)]decanyl, adamantyl, phenyl, naphthyl and anthracenyl. In these groups, one or more hydrogen atoms may be substituted by a heteroatom such as oxygen, sulfur, nitrogen or halogen, or one or more carbon atoms may be substituted by a heteroatom such as oxygen, sulfur or nitrogen, so that the group may contain a hydroxyl, cyano, carbonyl, ether, ester, sulfonic acid ester, carbonate, lactone ring, sultone ring, carboxylic anhydride or haloalkyl moiety.

Suitable divalent hydrocarbon groups include straight alkane-diyl groups such as methylene, ethylene, propane-1,3-diyl, butane-1,4-diyl, pentane-1,5-diyl, hexane-1,6-diyl, heptane-1,7-diyl, octane-1,8-diyl, nonane-1,9-diyl, decane-1,10-diyl, undecane-1,11-diyl, dodecane-1,12-diyl, tridecane-1,13-diyl, tetradecane-1,14-diyl, pentadecane-1,15-diyl, hexadecane-1,16-diyl, and heptadecane-1,17-diyl; saturated cyclic divalent hydrocarbon groups such as cyclopentanediyl, cyclohexanediyl, norbornanediyl and adamantanediyl; and unsaturated cyclic divalent hydrocarbon groups such as phenylene and naphthylene. In these groups, one or more hydrogen atom may be replaced by an alkyl moiety such as methyl, ethyl, propyl, n-butyl or t-butyl; one or more hydrogen atom may be replaced by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen; or a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene between carbon atoms, so that the group may contain a hydroxyl, cyano, carbonyl, ether, ester, sulfonic acid ester, carbonate, lactone ring, sultone ring, carboxylic anhydride or haloalkyl moiety. Of the heteroatoms, oxygen is preferred.

Of the PAGs having formula (2), those having formula (2′) are preferred.

In formula (2′), L^(A) is as defined above. R is hydrogen or trifluoromethyl, preferably trifluoromethyl. R³⁰¹, R³⁰² and R³⁰³ are each independently hydrogen or a C₁-C₂₀ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. Suitable monovalent hydrocarbon groups are as described above for R¹⁰⁵. The subscripts x and y are each independently an integer of 0 to 5, and z is an integer of 0 to 4.

Examples of the PAG having formula (2) are shown below, but not limited thereto. Notably, R is as defined above.

Of the foregoing PAGs, those having an anion of formula (1A′) or (1D) are especially preferred because of reduced acid diffusion and high solubility in the resist solvent. Also those having an anion of formula (2′) are especially preferred because of extremely reduced acid diffusion.

The PAG is preferably added in an amount of 0.1 to 50 parts, and more preferably 1 to 40 parts by weight per 100 parts by weight of the base polymer.

Other Components

With the sulfonium salt having formula (A), the base polymer, and the acid generator, all defined above, other components such as an organic solvent, surfactant, dissolution inhibitor, and crosslinker may be blended in any desired combination to formulate a chemically amplified positive or negative resist composition. This positive or negative resist composition has a very high sensitivity in that the dissolution rate in developer of the base polymer in exposed areas is accelerated by catalytic reaction. In addition, the resist film has a high dissolution contrast, resolution, exposure latitude, and process adaptability, and provides a good pattern profile after exposure, and minimal proximity bias because of restrained acid diffusion. By virtue of these advantages, the composition is fully useful in commercial application and suited as a pattern-forming material for the fabrication of VLSIs. Particularly when an acid generator is incorporated to formulate a chemically amplified positive resist composition capable of utilizing acid catalyzed reaction, the composition has a higher sensitivity and is further improved in the properties described above.

In the case of positive resist compositions, inclusion of a dissolution inhibitor may lead to an increased difference in dissolution rate between exposed and unexposed areas and a further improvement in resolution. In the case of negative resist compositions, a negative pattern may be formed by adding a crosslinker to reduce the dissolution rate of exposed area.

Examples of the organic solvent used herein are described in JP-A 2008-111103, paragraphs [0144]-[0145] (U.S. Pat. No. 7,537,880). Exemplary solvents include ketones such as cyclohexanone, cyclopentanone and methyl-2-n-pentyl ketone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, and 1-ethoxy-2-propanol; ethers such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propylene glycol dimethyl ether, and diethylene glycol dimethyl ether; esters such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, t-butyl acetate, t-butyl propionate, and propylene glycol mono-t-butyl ether acetate; and lactones such as γ-butyrolactone, which may be used alone or in admixture.

The organic solvent is preferably added in an amount of 100 to 10,000 parts, and ore preferably 200 to 8,000 parts by weight per 100 parts by weight of the base polymer.

Exemplary surfactants are described in JP-A 2008-111103, paragraphs [0165]-[0166]. Inclusion of a surfactant may improve or control the coating characteristics of the resist composition. The surfactant is preferably added in an amount of 0.0001 to 10 parts by weight per 100 parts by weight of the base polymer.

The dissolution inhibitor which can be used herein is a compound having at least two phenolic hydroxyl groups on the molecule, in which an average of from 0 to 100 mol % of all the hydrogen atoms on the phenolic hydroxyl groups are replaced by acid labile groups or a compound having at least one carboxyl group on the molecule, in which an average of 50 to 100 mol % of all the hydrogen atoms on the carboxyl groups are replaced by acid labile groups, both the compounds having a molecular weight of 100 to 1,000, and preferably 150 to 800. Typical are bisphenol A, trisphenol, phenolphthalein, cresol novolac, naphthalenecarboxylic acid, adamantanecarboxylic acid, and cholic acid derivatives in which the hydrogen atom on the hydroxyl or carboxyl group is replaced by an acid labile group, as described in U.S. Pat. No. 7,771,914 (JP-A 2008-122932, paragraphs [0155]-[0178]).

In the positive resist composition, the dissolution inhibitor is preferably added in an amount of 0 to 50 parts, more preferably 5 to 40 parts by weight per 100 parts by weight of the base polymer.

Suitable crosslinkers which can be used herein include epoxy compounds, melamine compounds, guanamine compounds, glycoluril compounds and urea compounds having substituted thereon at least one group selected from among methylol, alkoxymethyl and acyloxymethyl groups, isocyanate compounds, azide compounds, and compounds having a double bond such as an alkenyl ether group. These compounds may be used as an additive or introduced into a polymer side chain as a pendant. Hydroxy-containing compounds may also be used as the crosslinker.

Of the foregoing crosslinkers, examples of suitable epoxy compounds include tris(2,3-epoxypropyl) isocyanurate, trimethylolmethane triglycidyl ether, trimethylolpropane triglycidyl ether, and triethylolethane triglycidyl ether. Examples of the melamine compound include hexamethylol melamine, hexamethoxymethyl melamine, hexamethylol melamine compounds having 1 to 6 methylol groups methoxymethylated and mixtures thereof, hexamethoxyethyl melamine, hexaacyloxymethyl melamine, hexamethylol melamine compounds having 1 to 6 methylol groups acyloxymethylated and mixtures thereof. Examples of the guanamine compound include tetramethylol guanamine, tetramethoxymethyl guanamine, tetramethylol guanamine compounds having 1 to 4 methylol groups methoxymethylated and mixtures thereof, tetramethoxyethyl guanamine, tetraacyloxyguanamine, tetramethylol guanamine compounds having 1 to 4 methylol groups acyloxymethylated and mixtures thereof. Examples of the glycoluril compound include tetramethylol glycoluril, tetramethoxyglycoluril, tetramethoxymethyl glycoluril, tetramethylol glycoluril compounds having 1 to 4 methylol groups methoxymethylated and mixtures thereof, tetramethylol glycoluril compounds having 1 to 4 methylol groups acyloxymethylated and mixtures thereof. Examples of the urea compound include tetramethylol urea, tetramethoxymethyl urea, tetramethylol urea compounds having 1 to 4 methylol groups methoxymethylated and mixtures thereof, and tetramethoxyethyl urea.

Suitable isocyanate compounds include tolylene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate and cyclohexane diisocyanate. Suitable azide compounds include 1,1′-biphenyl-4,4′-bisazide, 4,4′-methylidenebisazide, and 4,4′-oxybisazide. Examples of the alkenyl ether group-containing compound include ethylene glycol divinyl ether, triethylene glycol divinyl ether, 1,2-propanediol divinyl ether, 1,4-butanediol divinyl ether, tetramethylene glycol divinyl ether, neopentyl glycol divinyl ether, trimethylol propane trivinyl ether, hexanediol divinyl ether, 1,4-cyclohexanediol divinyl ether, pentaerythritol trivinyl ether, pentaerythritol tetravinyl ether, sorbitol tetravinyl ether, sorbitol pentavinyl ether, and trimethylol propane trivinyl ether.

In the negative resist composition, the crosslinker is preferably added in an amount of 0.1 to 50 parts, more preferably 1 to 40 parts by weight per 100 parts by weight of the base polymer.

In the resist composition of the invention, a quencher other than the sulfonium salt having formula (A) may be blended. The other quencher is typically selected from conventional basic compounds. Conventional basic compounds include primary, secondary, and tertiary aliphatic amines, mixed amines, aromatic amines, heterocyclic amines, nitrogen-containing compounds with carboxyl group, nitrogen-containing compounds with sulfonyl group, nitrogen-containing compounds with hydroxyl group, nitrogen-containing compounds with hydroxyphenyl group, alcoholic nitrogen-containing compounds, amide derivatives, imide derivatives, and carbamate derivatives. Also included are primary, secondary, and tertiary amine compounds, specifically amine compounds having a hydroxyl, ether, ester, lactone ring, cyano, or sulfonic acid ester group as described in JP-A 2008-111103, paragraphs [0146]-[0164], and compounds having a carbamate group as described in JP 3790649. Addition of a basic compound may be effective for further suppressing the diffusion rate of acid in the resist film or correcting the pattern profile.

Onium salts such as sulfonium salts, iodonium salts and ammonium salts of sulfonic acids which are not fluorinated at α-position as described in U.S. Pat. No. 8,795,942 (JP-A 2008-158339) and similar onium salts of carboxylic acid may also be used as the other quencher. While an α-fluorinated sulfonic acid, imide acid, and methide acid are necessary to deprotect the acid labile group of carboxylic acid ester, an α-non-fluorinated sulfonic acid and a carboxylic acid are released by salt exchange with an α-non-fluorinated onium salt. An α-non-fluorinated sulfonic acid and a carboxylic acid function as a quencher because they do not induce deprotection reaction.

Also useful are quenchers of polymer type as described in U.S. Pat. No. 7,598,016 (JP-A 2008-239918). The polymeric quencher segregates at the resist surface after coating and thus enhances the rectangularity of resist pattern. When a protective film is applied as is often the case in the immersion lithography, the polymeric quencher is also effective for preventing a film thickness loss of resist pattern or rounding of pattern top. The other quencher is preferably added in an amount of 0 to 5 parts, more preferably 0 to 4 parts by weight per 100 parts by weight of the base polymer.

To the resist composition, a polymeric additive (or water repellency improver) may also be added for improving the water repellency on surface of a resist film as spin coated. The water repellency improver may be used in the topcoatless immersion lithography. Suitable water repellency improvers include polymers having a fluoroalkyl group and polymers having a specific structure with a 1,1,1,3,3,3-hexafluoro-2-propanol residue and are described in JP-A 2007-297590 and JP-A 2008-111103, for example. The water repellency improver to be added to the resist composition should be soluble in the organic solvent as the developer. The water repellency improver of specific structure with a 1,1,1,3,3,3-hexafluoro-2-propanol residue is well soluble in the developer. A polymer having an amino group or amine salt copolymerized as recurring units may serve as the water repellent additive and is effective for preventing evaporation of acid during PEB, thus preventing any hole pattern opening failure after development. An appropriate amount of the water repellency improver is 0 to 20 parts, preferably 0.5 to 10 parts by weight per 100 parts by weight of the base polymer.

Also, an acetylene alcohol may be blended in the resist composition. Suitable acetylene alcohols are described in JP-A 2008-122932, paragraphs [0179]-[0182]. An appropriate amount of the acetylene alcohol blended is 0 to 5 parts by weight per 100 parts by weight of the base polymer.

Process

The resist composition is used in the fabrication of various integrated circuits. Pattern formation using the resist composition may be performed by well-known lithography processes. The process generally involves coating, prebaking, exposure, post-exposure baking (PEB), and development. If necessary, any additional steps may be added.

For example, the positive resist composition is first applied onto a substrate on which an integrated circuit is to be formed (e.g., Si, SiO₂, SiN, SiON, TiN, WSi, BPSG, SOG, or organic antireflective coating) or a substrate on which a mask circuit is to be formed (e.g., Cr, CrO, CrON, MoSi₂, or SiO₂) by a suitable coating technique such as spin coating, roll coating, flow coating, dipping, spraying or doctor coating. The coating is prebaked on a hot plate at a temperature of 60 to 150° C. for 10 seconds to 30 minutes, preferably 80 to 120° C. for 30 seconds to 20 minutes. The resulting resist film is generally 0.01 to 2.0 μm thick.

The resist film is then exposed to a desired pattern of high-energy radiation such as UV, deep-UV, EB, EUV, x-ray, soft x-ray, excimer laser light, γ-ray or synchrotron radiation, directly or through a mask. The exposure dose is preferably about 1 to 200 mJ/cm², more preferably about 10 to 100 mJ/cm², or about 0.1 to 100 μC/cm², more preferably about 0.5 to 50 μC/cm². The resist film is further baked (PEB) on a hot plate at 60 to 150° C. for 10 seconds to 30 minutes, preferably 80 to 120° C. for 30 seconds to 20 minutes.

Thereafter the resist film is developed with a developer in the form of an aqueous base solution for 3 seconds to 3 minutes, preferably 5 seconds to 2 minutes by conventional techniques such as dip, puddle and spray techniques. A typical developer is a 0.1 to 10 wt %, preferably 2 to 5 wt % aqueous solution of tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), or tetrabutylammonium hydroxide (TBAH). The resist film in the exposed area is dissolved in the developer whereas the resist film in the unexposed area is not dissolved. In this way, the desired positive pattern is formed on the substrate. Inversely in the case of negative resist, the exposed area of resist film is insolubilized and the unexposed area is dissolved in the developer. It is appreciated that the resist composition of the invention is best suited for micro-patterning using such high-energy radiation as KrF and ArF excimer laser, EB, EUV, x-ray, soft x-ray, γ-ray and synchrotron radiation.

In an alternative embodiment, a negative pattern may be formed via organic solvent development using a positive resist composition comprising a base polymer having an acid labile group. The developer used herein is preferably selected from among 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, butenyl acetate, isopentyl acetate, propyl formate, butyl formate, isobutyl formate, pentyl formate, isopentyl formate, methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, pentyl lactate, isopentyl lactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzyl formate, phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, ethyl phenylacetate, and 2-phenylethyl acetate, and mixtures thereof.

At the end of development, the resist film is rinsed. As the rinsing liquid, a solvent which is miscible with the developer and does not dissolve the resist film is preferred. Suitable solvents include alcohols of 3 to 10 carbon atoms, ether compounds of 8 to 12 carbon atoms, alkanes, alkenes, and alkynes of 6 to 12 carbon atoms, and aromatic solvents. Specifically, suitable alcohols of 3 to 10 carbon atoms include n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol, 2-butyl alcohol, isobutyl alcohol, t-butyl alcohol, 1-pentanol, 2-pentanol, 3-pentanol, t-pentyl alcohol, neopentyl alcohol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol, cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol, 3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol, 2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol, 3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol, 4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol, cyclohexanol, and 1-octanol. Suitable ether compounds of 8 to 12 carbon atoms include di-n-butyl ether, diisobutyl ether, di-s-butyl ether, di-n-pentyl ether, diisopentyl ether, di-s-pentyl ether, di-t-pentyl ether, and di-n-hexyl ether. Suitable alkanes of 6 to 12 carbon atoms include hexane, heptane, octane, nonane, decane, undecane, dodecane, methylcyclopentane, dimethylcyclopentane, cyclohexane, methylcyclohexane, dimethylcyclohexane, cycloheptane, cyclooctane, and cyclononane. Suitable alkenes of 6 to 12 carbon atoms include hexene, heptene, octene, cyclohexene, methylcyclohexene, dimethylcyclohexene, cycloheptene, and cyclooctene. Suitable alkynes of 6 to 12 carbon atoms include hexyne, heptyne, and octyne. Suitable aromatic solvents include toluene, xylene, ethylbenzene, isopropylbenzene, t-butylbenzene and mesitylene. The solvents may be used alone or in admixture.

Rinsing is effective for minimizing the risks of resist pattern collapse and defect formation. However, rinsing is not essential. If rinsing is omitted, the amount of solvent used may be reduced.

A hole or trench pattern after development may be shrunk by the thermal flow, RELACS® or DSA process. A hole pattern is shrunk by coating a shrink agent thereto, and baking such that the shrink agent may undergo crosslinking at the resist surface as a result of the acid catalyst diffusing from the resist layer during bake, and the shrink agent may attach to the sidewall of the hole pattern. The bake is preferably at a temperature of 70 to 180° C., more preferably 80 to 170° C., for a time of 10 to 300 seconds. The extra shrink agent is stripped and the hole pattern is shrunk.

EXAMPLE

Examples of the invention are given below by way of illustration and not by way of limitation. The abbreviation “pbw” is parts by weight.

Sulfonium salts of iodized benzoic acid, designated Quenchers 1 to 29, used in resist compositions are identified below. Quenchers 1 to 29 were synthesized by ion exchange between an iodized benzoic acid providing the anion shown below and a sulfonium chloride providing the cation shown below.

SYNTHESIS EXAMPLE Synthesis of Base Polymers (Polymers 1 to 6)

Base polymers were prepared by combining suitable monomers, effecting copolymerization reaction thereof in tetrahydrofuran (THF) solvent, pouring the reaction solution into methanol for crystallization, repeatedly washing with hexane, isolation, and drying. The resulting polymers, designated Polymers 1 to 6, were analyzed for composition by ¹H-NMR spectroscopy, and for Mw and Mw/Mn by GPC versus polystyrene standards using THF solvent.

EXAMPLES AND COMPARATIVE EXAMPLES

Resist compositions were prepared by dissolving the polymer and selected components in a solvent in accordance with the recipe shown in Tables 1 to 4, and filtering through a filter having a pore size of 0.2 μm. The solvent contained 100 ppm of surfactant FC-4430 (3M). The components in Tables 1 to 4 are as identified below.

Organic Solvents:

PGMEA (propylene glycol monomethyl ether acetate)

GBL (γ-butyrolactone)

CyH (cyclohexanone)

PGME (propylene glycol monomethyl ether)

Acid Generators: PAG1 and PAG2 of the following Structural Formulae

Comparative Quenchers 1 to 7 of the following Structural Formulae

Water-Repellent Polymers 1 and 2

ArF Immersion Lithography Patterning Test Examples 1-1 to 1-23 and Comparative Examples 1-1 to 1-8

On a substrate (silicon wafer), a spin-on carbon film ODL-102 (Shin-Etsu Chemical Co., Ltd.) having a carbon content of 80 wt % was deposited to a thickness of 200 nm and a silicon-containing spin-on hard mask SHB-A940 having a silicon content of 43 wt % was deposited thereon to a thickness of 35 nm. On this substrate for trilayer process, each of the resist compositions in Tables 1 and 2 was spin coated, then baked on a hot plate at 100° C. for 60 seconds to form a resist film of 80 nm thick. Using an ArF excimer laser scanner NSR-S610C (Nikon Corp., NA 1.30, σ0.98/0.78, 35° cross-pole illumination, azimuthally polarized illumination), the resist film was exposed through a 6% halftone phase shift mask bearing a pattern having a line of 50 nm and a pitch of 100 nm (on-wafer size). The resist film was baked (PEB) at the temperature shown in Tables 1 and 2 for 60 seconds. Thereafter, the resist film was developed in n-butyl acetate for 30 seconds in Examples 1-1 to 1-13, 1-15 to 1-23 and Comparative Examples 1-1 to 1-7 or in 2.38 wt % tetramethylammonium hydroxide (TMAH) aqueous solution in Example 1-14 and Comparative Example 1-8, yielding a negative line-and-space (L/S) pattern having a space of 50 nm and a pitch of 100 nm.

The pattern was observed under a CD-SEM (CG-4000, Hitachi High-Technologies Corp.). The exposure dose capable of resolving a L/S pattern at 1:1 was determined as sensitivity, and edge roughness (LWR) was measured. The results are shown in Tables 1 and 2.

TABLE 1 Acid Organic PEB Polymer generator Quencher Water-repellent polymer solvent temp. Sensitivity LWR (pbw) (pbw) (pbw) (pbw) (pbw) (° C.) (mJ/cm²) (nm) Ex- 1-1 Polymer 1 PAG 1 Quencher 1 Water-repellent polymer 1 PGMEA (2,200) 95 42 3.1 am- (100) (8.0) (4.50) (4.0) GBL (300) ple 1-2 Polymer 1 PAG 1 Quencher 2 Water-repellent polymer 1 PGMEA (2,200) 95 39 3.2 (100) (8.0) (4.50) (4.0) GBL (300) 1-3 Polymer 1 PAG 1 Quencher 3 Water-repellent polymer 1 PGMEA (2,200) 95 44 3.9 (100) (8.0) (3.00) (4.0) GBL (300) 1-4 Polymer 1 PAG 1 Quencher 4 Water-repellent polymer 1 PGMEA (2,200) 95 48 2.8 (100) (8.0) (4.50) (4.0) GBL (300) 1-5 Polymer 1 PAG 1 Quencher 5 Water-repellent polymer 1 PGMEA (2,200) 95 34 3.6 (100) (8.0) (4.50) (4.0) GBL (300) 1-6 Polymer 1 PAG 1 Quencher 6 Water-repellent polymer 1 PGMEA (2,200) 95 36 3.1 (100) (8.0) (4.50) (4.0) GBL (300) 1-7 Polymer 1 PAG 1 Quencher 7 Water-repellent polymer 1 PGMEA (2,200) 95 46 3.0 (100) (8.0) (4.50) (4.0) GBL (300) 1-8 Polymer 1 PAG 1 Quencher 11 Water-repellent polymer 1 PGMEA (2,200) 95 37 2.8 (100) (8.0) (4.50) (4.0) GBL (300) 1-9 Polymer 1 PAG 1 Quencher 12 Water-repellent polymer 1 PGMEA (2,200) 95 39 2.7 (100) (8.0) (4.50) (4.0) GBL (300) 1-10 Polymer 1 PAG 1 Quencher 13 Water-repellent polymer 1 PGMEA (2,200) 95 39 2.6 (100) (8.0) (4.50) (4.0) GBL (300) 1-11 Polymer 1 PAG 1 Quencher 14 Water-repellent polymer 1 PGMEA (2,200) 95 38 2.7 (100) (8.0) (4.50) (4.0) GBL (300) 1-12 Polymer 2 — Quencher 11 Water-repellent polymer 1 PGMEA (2,200) 100 44 2.1 (100) (4.50) (4.0) GBL (300) 1-13 Polymer 3 PAG 1 Quencher 14 Water-repellent polymer 1 PGMEA (2,200) 90 42 2.3 (100) (6.0) (4.50) (4.0) GBL (300) PAG 2 (3.0) 1-14 Polymer 4 PAG 1 Quencher 5 Water-repellent polymer 2 PGMEA (2,200) 100 49 3.6 (100) (8.0) (4.50) (4.0) GBL (300) 1-15 Polymer 1 PAG 1 Quencher 15 Water-repellent polymer 1 PGMEA (2,200) 95 37 2.9 (100) (8.0) (4.50) (4.0) GBL (300) 1-16 Polymer 1 PAG 1 Quencher 16 Water-repellent polymer 1 PGMEA (2,200) 95 37 2.6 (100) (8.0) (4.50) (4.0) GBL (300) 1-17 Polymer 1 PAG 1 Quencher 17 Water-repellent polymer 1 PGMEA (2,200) 95 36 2.8 (100) (8.0) (4.50) (4.0) GBL (300) 1-18 Polymer 1 PAG 1 Quencher 18 Water-repellent polymer 1 PGMEA (2,200) 95 38 2.6 (100) (8.0) (4.50) (4.0) GBL (300) 1-19 Polymer 1 PAG 1 Quencher 19 Water-repellent polymer 1 PGMEA (2,200) 95 33 3.5 (100) (8.0) (2.50) (4.0) GBL (300) 1-20 Polymer 1 PAG 1 Quencher 20 Water-repellent polymer 1 PGMEA (2,200) 95 31 3.0 (100) (8.0) (3.50) (4.0) GBL (300) 1-21 Polymer 1 PAG 1 Quencher 21 Water-repellent polymer 1 PGMEA (2,200) 95 36 3.2 (100) (8.0) (4.50) (4.0) GBL (300) 1-22 Polymer 1 PAG 1 Quencher 22 Water-repellent polymer 1 PGMEA (2,200) 95 35 2.3 (100) (8.0) (5.50) (4.0) GBL (300) 1-23 Polymer 1 PAG 1 Quencher 23 Water-repellent polymer 1 PGMEA (2,200) 95 38 2.1 (100) (8.0) (5.50) (4.0) GBL (300)

TABLE 2 Acid Organic PEB Polymer generator Quencher Water-repellent polymer solvent temp. Sensitivity LWR (pbw) (pbw) (pbw) (pbw) (pbw) (° C.) (mJ/cm²) (nm) Com- 1-1 Polymer 1 PAG1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 37 5.0 par- (100) (8.0) Quencher 1 (4.0) GBL (300) ative (2.13) Ex- 1-2 Polymer 1 PAG 1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 46 4.4 am- (100) (8.0) Quencher 2 (4.0) GBL (300) ple (2.13) 1-3 Polymer 1 PAG 1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 38 4.3 (100) (8.0) Quencher 3 (4.0) GBL (300) (4.50) 1-4 Polymer 1 PAG 1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 42 4.5 (100) (8.0) Quencher 4 (4.0) GBL (300) (4.50) 1-5 Polymer 1 PAG 1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 56 4.9 (100) (8.0) Quencher 5 (4.0) GBL (300) (4.50) 1-6 Polymer 1 PAG 1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 39 4.6 (100) (8.0) Quencher 6 (4.0) GBL (300) (4.50) 1-7 Polymer 1 PAG 1 Comparative Water-repellent polymer 1 PGMEA (2,200) 95 31 4.8 (100) (8.0) Quencher 7 (4.0) GBL (300) (4.50) 1-8 Polymer 4 PAG 1 Comparative Water-repellent polymer 2 PGMEA (2,200) 100 58 6.0 (100) (8.0) Quencher 7 (4.0) GBL (300) (4.50)

EB Lithography Test Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3

Each of the resist compositions in Table 3 was spin coated onto a silicon substrate, which had been vapor primed with hexamethyldisilazane (HMDS), and pre-baked on a hot plate at 110° C. for 60 seconds to form a resist film of 80 nm thick. Using a system HL-800D (Hitachi Ltd.) at an accelerating voltage of 50 kV, the resist film was exposed imagewise to EB in a vacuum chamber. Immediately after the image writing, the resist film was baked (PEB) on a hot plate at 80° C. for 60 seconds and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds to form a pattern.

The resist pattern was evaluated for sensitivity, resolution and LWR. The resolution is a minimum L/S size at the exposure dose that provides a resolution as designed of a 120-nm L/S pattern. The 120-nm L/S pattern was measured for LWR. The results are shown in Table 3.

TABLE 3 Acid Organic Polymer generator Quencher solvent Sensitivity Resolution LWR (pbw) (pbw) (pbw) (pbw) (μC/cm²) (nm) (nm) Example 2-1 Polymer 5 — Quencher 8 PGMEA (400) 35 80 3.4 (100) (2.50) CyH (2,000) PGME (100) 2-2 Polymer 5 — Quencher 9 PGMEA (400) 36 80 3.0 (100) (2.50) CyH (2,000) PGME (100) 2-3 Polymer 5 — Quencher 10 PGMEA (400) 39 80 3.1 (100) (2.50) CyH (2,000) PGME (100) Com 2-1 Polymer 5 — Comparative PGMEA (400) 38 90 5.0 parative (100) Quencher 1 CyH (2,000) Example (2.50) PGME (100) 2-2 Polymer 5 — Comparative PGMEA (400) 38 90 5.2 (100) Quencher 2 CyH (2,000) (2.50) PGME (100) 2-3 Polymer 5 — Comparative PGMEA (400) 38 90 5.5 (100) Quencher 3 CyH (2,000) (2.50) PGME (100)

EUV Lithography Test Examples 3-1 to 3-9 and Comparative Example 3-1

The resist composition in Table 4 was spin coated on a silicon substrate having a 13-nm coating of SOG material SHB-A940 (Shin-Etsu Chemical Co., Ltd.) and prebaked on a hot plate at 105° C. for 60 seconds to form a resist film of 60 nm thick. Using an EUV scanner NXE3300 (ASML, NA 0.33, σ0.9/0.6, quadrupole illumination), the resist film was exposed to EUV through a patterned mask. The resist film was baked (PEB) at the temperature shown in Table 4 and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds to form a hole pattern having a size of 23 nm and a pitch of 46 nm.

The resist pattern was evaluated. The exposure dose that provides a hole pattern having a size of 23 nm is reported as sensitivity. The size of 50 holes was measured under CD-SEM (CG-5000, Hitachi High-Technologies Corp.), from which a size variation (3σ) was computed and reported as CDU.

The resist composition is shown in Table 4 together with the sensitivity and CDU of EUV lithography.

TABLE 4 Acid Organic Polymer generator Quencher solvent PEB temp. Sensitivity CDU (pbw) (pbw) (pbw) (pbw) (° C.) (mJ/cm²) (nm) Example 3-1 Polymer 6 — Quencher 20 PGMEA (400) 100 24 2.4 (100) (3.50) CyH (2,000) PGME (100) 3-2 Polymer 6 — Quencher 22 PGMEA (400) 100 20 2.3 (100) (3.50) CyH (2,000) PGME (100) 3-3 Polymer 6 — Quencher 23 PGMEA (400) 100 22 2.3 (100) (3.50) CyH (2,000) PGME (100) 3-4 Polymer 6 — Quencher 24 PGMEA (400) 100 20 2.3 (100) (3.50) CyH (2,000) PGME (100) 3-5 Polymer 6 — Quencher 25 PGMEA (400) 100 18 2.3 (100) (3.50) CyH (2,000) PGME (100) 3-6 Polymer 6 — Quencher 26 PGMEA (400) 100 17 2.3 (100) (3.50) CyH (2,000) PGME (100) 3-7 Polymer 6 — Quencher 27 PGMEA (400) 100 18 2.2 (100) (3.50) CyH (2,000) PGME (100) 3-8 Polymer 6 — Quencher 28 PGMEA (400) 100 19 2.3 (100) (3.50) CyH (2,000) PGME (100) 3-9 Polymer 6 — Quencher 29 PGMEA (400) 100 22 2.4 (100) (3.50) CyH (2,000) PGME (100) Comparative 3-1 Polymer 6 — Comparative PGMEA (400) 100 38 4.0 Example (100) Quencher 6 (2.50) CyH (2,000) PGME (100)

It is demonstrated in Tables 1 to 4 that resist compositions comprising a sulfonium salt having formula (A) within the scope of the invention offer a satisfactory resolution and improved LWR or CDU.

Japanese Patent Application No. 2016-113475 is incorporated herein by reference. Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A resist composition comprising a base polymer and a sulfonium salt having the formula (A):

wherein R¹ is hydroxyl, X is a single bond, R², R³ and R⁴ are each independently fluorine, chlorine, bromine, iodine, or a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain an oxo moiety, a C₂-C₁₂ straight, branched or cyclic alkenyl group which may contain an oxo moiety, C₆-C₂₀ aryl group, C₇-C₁₂ aralkyl group or C₇-C₁₂ aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R² and R³ may bond together to form a ring with the sulfur atom to which they are attached, m is an integer of 1 to 5, n is an integer of 1 to 3, and p is an integer of 1 to
 3. 2. The resist composition of claim 1 wherein m is equal to
 3. 3. The resist composition of claim 1, further comprising an acid generator capable of generating a sulfonic acid, imide acid or methide acid.
 4. The resist composition of claim 1, further comprising an organic solvent.
 5. The resist composition of claim 1 wherein the base polymer comprises recurring units having the formula (a1) or recurring units having the formula (a2):

wherein R^(A) is each independently hydrogen or methyl, Y¹ is a single bond, ester group, C₁-C₁₂ linking group containing lactone ring, phenylene group or naphthylene group, Y² is a single bond or ester group, R¹¹ and R¹² each are an acid labile group.
 6. The resist composition of claim 5, further comprising a dissolution inhibitor.
 7. The resist composition of claim 5 which is a chemically amplified positive resist composition.
 8. The resist composition of claim 1 wherein the base polymer is free of an acid labile group.
 9. The resist composition of claim 8, further comprising a crosslinker.
 10. The resist composition of claim 8 which is a chemically amplified negative resist composition.
 11. The resist composition of claim 1, further comprising a surfactant.
 12. The resist composition of claim 1 wherein the base polymer further comprises recurring units of at least one type selected from the formulae (f1) to (f3):

wherein R^(A) is each independently hydrogen or methyl, R²¹ is a single bond, phenylene group, —O—R³¹—, or —C(═O)-Q¹-R³¹—, Q¹ is —O— or —NH—, R³¹ is a C₁-C₆ straight, branched or cyclic alkylene group, C₂-C₆ straight, branched or cyclic alkenylene group, or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety, R²² to R²⁹ are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C₆-C₁₂ aryl group, C₇-C₂₀ aralkyl group or mercaptophenyl group, Z¹ is a single bond, —-Z¹¹—C(═O)—O—, —Z¹¹—O— or —Z¹¹—O—C(═O)—, Z¹¹ is a C₁-C₁₂ straight, branched or cyclic alkylene group which may contain a carbonyl, ester or ether moiety, A is hydrogen or trifluoromethyl, Z² is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R³²—, or —C(═O)—Z¹²—R³²—, Z¹² is —O— or —NH—, R³² is a C₁-C₆ straight, branched or cyclic alkylene group, phenylene group, fluorinated phenylene group, trifluoromethyl-substituted phenylene group, or C₂-C₆ straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxy moiety, M⁻ is a non-nucleophilic counter ion.
 13. A process for forming a pattern comprising the steps of applying the resist composition of claim 1 onto a substrate, baking to form a resist film, exposing the resist film to high-energy radiation, and developing the exposed film in a developer.
 14. The process of claim 13 wherein the high-energy radiation is ArF excimer laser radiation of wavelength 193 nm or KrF excimer laser radiation of wavelength 248 nm.
 15. The process of claim 13 wherein the high-energy radiation is electron beam or extreme ultraviolet radiation of wavelength 3 to 15 nm.
 16. A sulfonium salt having the formula (A-1):

wherein R^(1′) is hydroxyl, R^(2′), R^(3′) and R^(4′) are each independently a C₁-C₁₂ straight, branched or cyclic alkyl group which may contain an oxo moiety, a C₂-C₁₂ straight, branched or cyclic alkenyl group which may contain an oxo moiety, C₆-C₂₀ aryl group, C₇-C₁₂ aralkyl group or C₇-C₁₂ aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R^(2′) and R^(3′) may bond together to form a ring with the sulfur atom to which they are attached. 